A review on continuous biomass hydrothermal liquefaction systems: Process design and operating parameters effects on biocrude

In the era of limited energy resources, researchers and scientists are highlighting bioenergy or biomass energy as a sustainable form of renewable and upgradable energy source. Hydrothermal liquefaction, known as HTL, is a thermochemical conversion process which produces mainly biocrude, from several biomass types: microalgae, waste water, crops, etc. Commercializing HTL and upscaling it for industrial applications needs intensive studies for building continuous systems. This work defines the HTL of biomass, including the major components of a continuous system: feeding system, heating system, reactor and separator. Many previous studies are compared to investigate the effect of operating parameters; residence time, temperature, pressure, flow rate, catalyst and co-solvent types on the products' qualities and quantities. In addition, using data from previous HTL studies, a new analytic study concerning the effect of the biomass composition on the optimum temperature of liquefaction and the maximum yield of biocrude is presented and the results show the effect of the biomass used on these values. Although, using previous works of continuous HTL systems, this work presents all the aspects of continuous processing, including the hardware used and the parameters involved in the hydrothermal processing of biomass. In addition, a transition phase from batch to continuous processing, re-defining the variables needed for the design, is proposed. The analysis of the biomass components and the operating parameters effects, and, the proposed transition phase will be used to select and calibrate the suitable conditions for designing and constructing a large scale continuous HTL system.